1. Field of the Invention
The invention relates to voltage convertors and, more particularly, to a converter for converting an input-side alternating voltage into an output-side direct voltage, where a power factor correction is provided and the converter comprises a transformer having at least two primary windings arranged in series. The invention also relates to a method for operating such a converter.
2. Description of the Related Art
Converters used in conventional commercial or mains power supplies usually have extremely harmonically polluted input currents or, as the case may be, a power factor that is significantly under unity. The reason for this lies in the requirement to recharge a storage capacitor on the direct current side by a rectifier on the alternating current side. As a result, short, needle-shaped current peaks are created. Without additional measures, the height of the current peaks is limited only by the internal resistances of an input-side power grid, a power grid line filter, a rectifier and a storage capacitor.
According to the prior art, a technique known as power factor correction (PFC) is provided to reduce a fraction of interfering harmonic components in the current draw of a converter.
Passive power factor correction is achieved by a high input inductance (FIG. 1). The large inductance value is necessary to increase the current flow angle significantly during a recharging phase. This makes sense only in the case of small power ratings, because otherwise the corresponding chokes will be too large and heavy. Apart from the costs of the chokes, their power dissipation loss needs to be taken into account.
This method is little suited for wide ranges of input voltage due to the variance of the maximum input current associated therewith.
Alternatively thereto, an active form of power factor correction is known in which a separate converter stage correctively adjusts the consumed current to the time characteristic of a sinusoidally varying power supply voltage. Such active PFC circuits are generally embodied as step-up converters and are connected directly downstream of a rectifier (FIG. 2a). They charge a large capacitor to a voltage above the peak voltage of the input alternating voltage. The step-up converter operates at a considerably higher frequency than a power grid supply, as a result of which a much smaller inductance is required. An approximately continuous current flow is produced with a low current ripple factor, the average current being adjusted by a control circuit to the value of the power supply voltage at a given instant in time.
Although an active PFC circuit is more complex and costly than passive power factor correction, higher levels of efficiency and superior harmonic suppression are possible. A disadvantageous aspect apart from the cost and complexity is that, due to its operating principle, the output voltage of such a PFC circuit lies above the maximum power supply voltage. As a result, problems can occur in relation to component loads and insulation voltages, especially at high power supply input voltages.
Instead of a step-up converter, an active PFC circuit can include a step-down converter to which an output voltage less than the power supply voltage is applied (FIG. 2b). This does, however, lead to a reduction in the possible current flow angle. The energy input into a storage capacitor can only be accomplished with a power supply voltage that is greater than the voltage at the storage capacitor. Furthermore, the current ripple factor is higher than in the case of a solution comprising a step-up converter and it is easier to control a ground-side power switch in the case of a step-up converter.
JP 2002 315 327 A discloses a flyback converter having a transformer which comprises two primary-side windings. This converter implementation is likewise directed toward power factor correction. A connecting point between the primary-side windings is connected to a reference potential by a smoothing capacitor. In this arrangement, the first primary winding is arranged in series with the smoothing capacitor and connected to a rectified input alternating voltage. The second primary winding is connected in series with a switch at the smoothing capacitor. When the switch is turned on, energy is initially loaded from the smoothing capacitor into the transformer by the second primary winding, with a voltage also building up in the first primary winding. After the switch is turned off, energy from the input is initially stored in the smoothing capacitor by the first primary winding and then the energy still stored in the transformer is output on the secondary side. The input current is regulated in this case by the transformer saturation. In addition to having a narrow input voltage range, a converter of this type is subject to undesirable losses due to the charge reversal operations.